In thin film batteries, a solid state electrolyte is disposed between an anode and cathode. A suitable electrolyte comprises lithium phosphorous oxynitride material which may be deposited on a substrate by radio frequency sputtering of a lithium phosphate target in a nitrogen gas, as for example, disclosed in U.S. Pat. No. 5,597,660, and U.S. Pat. No. 5,512,147 which are incorporated herein by reference in their entireties. In the deposition process, a sufficient amount of nitrogen species have to be incorporated into the electrolyte to provide a good Li ion conductivity and good chemical stability. Because the incorporation efficiency of nitrogen is relatively low, the electrolyte is typically deposited in a relatively high pressure nitrogen atmosphere, typically at pressures of 20 mTorr or higher. However, the high nitrogen gas pressure lowers the deposition rates obtained during the sputtering process. The high gas pressure may also render the plasma relatively unstable and difficult to control.
Thus it is desirable to obtain good deposition rates for depositing lithium phosphorous oxynitride material. It is also desirable to obtain a stable and easy to control plasma during the sputtering process.